runtime: use a C function to call mmap

The final argument to mmap, of type off_t, varies.
In CL 445375 we changed it to always use the C off_t type,
but that broke 32-bit big-endian Linux systems.  On those systems,
using the C off_t type requires calling the mmap64 function.
In C this is automatically handled by the <sys/mman.h> file.
In Go, we would have to change the magic //extern comment to
call mmap64 when appropriate.  Rather than try to get that right,
we instead go through a C function that uses C implicit type
conversions to pick the right type.

Fixes PR go/110297

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/504415

1 files changed, 21 insertions, 0 deletions

diff --git a/libgo/runtime/go-mmap.c b/libgo/runtime/go-mmap.c
new file mode 100644
index 0000000..b2327ba
--- /dev/null
+++ b/libgo/runtime/go-mmap.c
@@ -0,0 +1,21 @@
+/* go-mmap.c -- functions for calling C mmap functions.
+
+   Copyright 2023 The Go Authors. All rights reserved.
+   Use of this source code is governed by a BSD-style
+   license that can be found in the LICENSE file.  */
+
+#include "config.h"
+
+#include <stdint.h>
+#include <sys/mman.h>
+
+/* The exact C function to call varies between mmap and mmap64, and
+   the size of the off_t argument also varies.  Here we provide a
+   function that Go code can call with consistent types.  */
+
+void *
+__go_mmap(void *addr, uintptr_t length, int32_t prot, int32_t flags,
+	  int32_t fd, uintptr_t offset)
+{
+  return mmap(addr, length, prot, flags, fd, offset);
+}